1. Field of the Invention
The present invention relates to a photoelectric converter, its driving method, and a system including the photoelectric converter. More particularly, the present invention relates to a one-dimensional or two-dimensional photoelectric converter, its driving method, and a system including the photoelectric converter which can read the same size of original documents such as those from, for example, a facsimile, a digital copying machine, or an X-ray camera.
2. Related Background Art
Conventionally, a read system having a condensed optical system and a CCD-type sensor has been used as a read system such as a facsimile, a digital copying machine, or an X-ray camera. In recent years, however, a development of photoelectric converting semiconductor materials represented by hydrogenated amorphous-silicon (hereinafter "a-Si") has contributed to an advancement of developing so-called a con act-type sensor in which a photoelectric converting element and a signal processor are formed on a large-sized substrate to read the same size of copies as for an information source by using a photoelectric system, and it has been or is being put to practical use. Particularly, a-Si can be used not only as photoelectric converting material, but also as semiconductor material for thin film electric field effect type transistor (hereinafter "TFT"), therefore, photoelectric converting semiconductor layer and a TFT semiconductor layer can be formed conveniently.
FIGS. 1A and 1B are typical sectional views each of which is used to show an example of a structure of a conventional optical sensor, in other words, an example of a layer structure of the optical sensor, and FIG. 1C is a schematic circuit diagram used to describe a driving method, which shows an example of a typical driving method available for both FIGS. 1A and 1B. Each of FIGS. 1A and 1B shows a photodiode type optical sensor; the structure in FIG. 1A is called a PIN type, and that in FIG. 1B is called a Schottky type. In FIGS. 1A and 1B, reference numerals 1, 2, 3, 4, and 5 indicate an insulating substrate, a lower electrode, a p type semiconductor layer (hereinafter "p-layer"), an intrinsic semiconductor (hereinafter "i-layer"), an n type semiconductor (hereinafter "n-layer"), and a transparent electrode, respectively. In the Schottky type structure in FIG. 1B, materials for the lower electrode 2 are appropriately selected to form a Schottky barrier layer so that unnecessary electrons will not be injected from the lower electrode 2 to the i-layer 4.
In FIG. 1C, reference numerals 10, 11, and 12 indicate the symbolized above optical sensor, a power supply, and a detector of a current amplifier or the like, respectively. In the optical sensor 10, a direction shown by C indicates a side of the transparent electrode 6 in FIGS. 1A and 1B, a direction shown by A indicates a side of the lower electrode 2, and the power supply 11 is set so that a positive voltage is applied to side C across side A. Now, the operation is roughly described below.
As shown in FIGS. 1A and 1B, light is incident from a direction shown by an arrow. When the light reaches the i-layer 4, it is absorbed and electrons and holes are generated. Since an electric field is applied to the i-layer 4 by the power supply 11, the electrons move to the side C, in other words, they move to the transparent electrode 6 after passing through the n-layer 5, and the holes move to the side A, in other words, to the lower electrode 2. Accordingly, optical current is fed to the optical sensor 10. If light is not incident on the layer, electrons and holes are not generated on the i-layer 4; for the holes in the transparent electrode 6, the n-layer 5 acts as a hole injection blocking layer, and for electrons in the lower electrode 2, the p-layer 3 in the PIN type structure in FIG. 1A or the Schottky barrier layer in the Schottky type structure in FIG. 1B acts as an electron injection blocking layer, therefore, both the electrons an d holes cannot move and no current is applied. As described above, the presence or absence of the incident light varies the current fed to a circuit. If the change is detected by the detector 12 in FIG. 1C, the layers act as an optical sensor.
For the above conventional optical sensor, however, it is difficult to produce a high signal-to-noise ratio and low cost photoelectric converter. The reasons are described below.
The first reason is that the injection blocking layer is required at two portions both in the PIN type structure in FIG. 1A and the Schottky type structure in FIG. 1B.
In the PIN type structure in FIG. 1A, the n-layer 5 which is an injection blocking layer requires not only introducing electrons to the transparent electrode 6 and but also inhibiting holes from being injected to the i-layer 4. If the layer loses one of those characteristics, the optical current may decrease or increase due to current generated without incident light (hereinafter "dark current"), which leads to lowering the signal-to-noise ratio. The dark current itself can be considered as a noise and also includes fluctuation called a shot noise, in other words, quantization noise, therefore, the quantization noise in the dark current cannot be reduced even if the dark current is removed by the detector 12.
Generally, to improve those characteristics, it is required to optimize conditions of creating films for the i-layer 4 and n-layer 5 and conditions of annealing after the creation. Also for the p-layer 3 which is another injection blocking layer, however, the equivalent characteristics are required even though electrons and holes are reversed, and the both conditions must be optimized in the same manner. In general, the optimizing conditions for the former n-layer are not the same as for the p-layer, and it is hard to satisfy both conditions simultaneously.
In other words, if the injection blocking layer is required at two portions in the same optical sensor, it is difficult to form an optical sensor having high signal-to-noise ratio.
This can also be said about the Schottky type structure shown in FIG. 1B. Additionally, in the Schottky type structure in FIG. 1B, a Schottky barrier layer is used for one injection blocking layer, in which a difference between work functions of the lower electrode 2 and the i-layer 4 is used, therefore, materials for the lower electrode 2 are restricted or the characteristics are largely affected by localized levels of an interface and it is even more difficult to satisfy the conditions.
It is also reported that approximately 100 .ANG. of a thin silicon or a metal oxide or nitride film is formed between the lower electrode 2 and the i-layer 4 to further improve the characteristics of the Schottky barrier layer. In this method, however, holes are introduced to the lower electrode 2 by using a tunneling effect to enhance an effect of inhibiting electrons from being injected to the i-layer 4 and a difference between work functions is also used, therefore, materials for the lower electrode 2 must be restricted. In addition, since it uses contrary characteristics, i.e., blocking injection of the electrons and movement of the holes caused by the tunneling effect, the oxide or nitride film must be extremely thin, i.e., approximately 100 .ANG.. The control of the thickness and layer features is difficult and reduces productivity.
Further, the requirement of two portions of the injection blocking layer not only reduces productivity, but also increases cost. This is because desired characteristics for an optical sensor cannot be obtained if a problem is caused by dust even at a single portion of the injection blocking layer, since the injection blocking layer is important as its characteristics.
By using FIG. 2, the second reason is described below. FIG. 2 shows a layer structure of an electric field effect type transistor (TFT) formed by thin semiconductor films. The TFT is sometimes used as a part of a control section to form a photoelectric converter. In this drawing, the same parts as for FIGS. 1A to 1C are designated by corresponding reference numerals. In FIG. 2, reference numerals 7 and 60 indicate a gate insulating film and an upper electrode, respectively. How to form them is described in order. A lower electrode 2 acting as a gate electrode (G), a gate insulating film 7, an i-layer 4, an n-layer 5, and upper electrodes 60 acting as source and drain electrodes (S, D) are laid on an insulating substrate 1 in this order, and an etching process is made for the upper electrodes 60 to form the source and drain electrodes, then for the n-layer 5 to form a channel section. The TFT has characteristics of being sensitive to a state of the interface of the gate insulating film 7 and the i-layer 4, and generally they are laid repeatedly in the same vacuum to inhibit them from being contaminated.
When the conventional optical sensor is formed on the same substrate as for the TFT, this layer structure has a problem, which may increase cost or reduce its characteristics. This is because the conventional optical sensor shown in FIGS. 1A to 1C has a structure of an electrode, a p-layer, an i-layer, an n-layer, and an electrode in the PIN type structure in FIG. 1A and an electrode, an i-layer, an n-layer, and an electrode in the Schottky type structure in FIG. 1B, while the TFT has a structure of an electrode, an insulating film, an i-layer, an n-layer, and an electrode. Therefore, their layer structures are not identical. It indicates that the optical sensor and the TFT cannot be formed by the same process at a time, and a complicated process may lower a yielding ratio or increase cost due to repetition of a photolitho process since a required layer is formed at a required place. In addition, to make the i-layer and the n-layer identical in both structures, an etching process for the gate insulating film 7 and the p-layer 3 is required, which may cause a problem that in the same vacuum it is impossible to accumulate the injection blocking layers, the p-layer 3 and the i-layer 4 which are important for the optical sensor as described in the above or that the interface of the important gate insulating film 7 and i-layer 4 of the TFT is contaminated by the etching process for the gate insulating film which may leads to deteriorating the characteristics or lowering signal-to-noise ratio.
Although the order of the layer structure is identical for the above sensor in which an oxide or nitride film is laid between the lower electrode 2 and the i-layer 4 to improve the characteristics of the Schottky type structure in FIG. 1B, the oxide film and the nitride film must have a thickness of approximately 100 .ANG. as described above, and it is difficult that they are used with the gate insulating film. FIG. 3 shows a result of our experiment on the gate insulating film and the TFT yielding ratio. The yielding ratio is rapidly reduced in the range of 1,000 .ANG. or less of the thickness of the gate insulating film; the yielding ratio is approximately 30% at 800 .ANG., approximately 0% at 500.ANG., and at 250 .ANG., the TFT operation could not be even confirmed. Accordingly, it is apparently difficult to use the oxide film or the nitride film of the optical sensor for which the tunneling effect is used and the gate insulating film of the TFT which requires insulation from electrons and holes together, as shown by this data.
Furthermore, it is difficult to create a capacitance element (hereinafter "capacitor"), which is an element (not shown) needed for obtaining integrated values of electric charge or current, having good characteristics of a small quantity of leakage in the same structure as for the conventional optical sensor. It is because the capacitor is used for accumulating electric charges between two electrodes, therefore, it always requires a layer for blocking movement of electrons and holes in the middle layer between electrodes, while in the conventional optical sensor only a semiconductor layer is used between the electrodes, therefore, it is hard to obtain a middle layer having good characteristics with a small quantity of thermal leak.
The poor matching between the TFT and the capacitor, which are important elements to form the photoelectric converter in processes or as characteristics, requires one-dimensional or two-dimensional arrangement of multiple optical sensors. This leads to increased and complicated processes in composing an entire system which detects its optical signals sequentially and therefore to extremely low yielding ratio. Accordingly, it may be a serious problem to create a high-performance and multifunctional device at low cost.